The use of sulfur and phosphorus-containing compounds as anti-wear additives in lubricant compositions is known. In general, the sulfur and phosphorus-containing compounds comprise alkyl acid phosphates, alkyl acid thiophosphates, alkyl acid dithiophosphates, and their amine or metal salts. For example, zinc dialkyldithiophosphate (ZDDP) is a well-known sulfur and phosphorus-containing anti-wear agent. Because zinc is an environmental contaminant, the industry is under pressure to find novel metal-free (i.e. ashless) antiwear additives. In addition, because these compounds have acidic functionality, the compounds may actually attack the surface of the metal parts being lubricated and thus cause greater wear or corrosion.
As automobile manufacturers continue to make larger trucks with more powerful engines the amount of torque applied to the axles of these vehicles has increased enormously. Accordingly, the Original Equipment Manufacturers (OEM) have placed increased demands on the lubricant industry to address the increase in torque and extend axle life. In particular, what is needed is a lubricant composition having improved antiwear, thermal stability, and oxidative stability. Furthermore, there is needed a lubricant composition that can provide improved antiwear and thermal stability as evidenced by automotive screening tests such as the L-37 test ASTM D-6121.
Finally, a more recent problem attributed to known sulfur and phosphorus-containing antiwear compounds relates to their toxicity in lubricant compositions. Due to their acidic nature, such compounds may attack metal parts lubricated with fluids containing the compounds and thereby cause greater wear and/or corrosion. Accordingly, these compounds are typically neutralized with an oil-soluble amine. The neutralizing amines currently used for this purpose are particularly toxic, persistent and bioaccumulative. An amine-free, sulfur and phosphorus-containing compound is therefore desirable as an environmentally friendly alternative to today's anti-wear additives.
It has now been discovered that certain novel compounds as described below may be readily formulated into lubricating compositions to afford a unique solution for providing desired lubricant performance characteristics, such as improved anti-wear performance and good thermal stability while maintaining a low toxicity environmental footprint.